1. Field of the Invention
The invention relates to circuit breaker circuit protection devices for electrical distribution systems. More particularly the present invention is directed to latch mechanisms for tripping the operating mechanism of a circuit breaker in response to an actual fault detection made by either a thermal-magnetic electromechanical or electronic trip unit (or other electronic monitoring device) that operate independently within a circuit breaker. Alternatively the operating mechanism may be tripped in response to simulated fault detection in the distribution system.
2. Description of the Prior Art
Circuit breakers are utilized in electrical distribution systems to interrupt power current flow upon detection of a potential fault in the system. Generally circuit breakers are interposed in a power distribution circuit between a line source of power and a downstream circuit load. A circuit breaker commonly includes one or more fixed and moving separable contact pairs that open and close the power distribution circuit. A trip unit (often thermal-magnetic electromechanical, analog electronic, digital electronic or combination) monitors circuit load and causes an operating mechanism to separate the contact pair (open the circuit) upon detection of a fault condition. Examples of distribution system faults include short circuit or thermal overheating overloads, ground faults and arc faults.
Circuit breakers incorporating both a thermal-magnetic electromechanical overload detection trip unit and an electronic fault interruption unit that operate independently within the circuit breaker are sold in the United States of America by Siemens Energy & Automation, Inc. (“Siemens”) and other companies. An exemplary Siemens circuit breaker is shown in FIGS. 1-3. The Siemens circuit breaker incorporates a thermal-magnetic electromechanical trip unit for detection of short circuit and over current faults in electric power distribution circuits, and also an independently operating electronic fault interruption unit for detection of arc fault, ground fault or combination of both types of faults. Both the electromechanical trip unit and electronic fault interruption unit need to be able to activate the operating mechanism independently to open the circuit breaker contacts upon fault detection by either respective unit.
As shown in FIG. 1, the circuit breaker 10 is connected to a power source such as the line stab 11 of a power panel by sliding connection with the line terminal 12. A power panel neutral terminal 13 is connected to the circuit breaker panel neutral wire 14. The circuit breaker 10 load power terminal 15 is connected to load circuit power wire 16. Correspondingly, the circuit breaker 10 load neutral terminal 17 is connected to the load circuit neutral wire 18.
The circuit breaker 10 has a multi-component housing 20, including a base 20A, intermediate cover 20B and top cover 20C. The base 20A and intermediate cover 20B form a first compartment. The intermediate cover 20B and top cover 20C in turn form a second compartment. The circuit breaker handle 22 allows an operator to energize and de-energize the electrical distribution circuit, as well as reset the circuit breaker after fault condition trips the circuit breaker. The exemplary Siemens circuit breaker also has an electronic trip indicator light 24 and a test button 26 that is used to simulate a fault and confirm the breaker 10 operating condition. The fault circuit interrupter 27 is shown schematically and is of known design. The circuit breaker housing components 20A, 20B and 20C are held together in tandem by a plurality of rivets 28, one of which is shown.
FIG. 2 shows a schematic plan view of the first compartment of the known Siemens circuit breaker 10, showing exemplary components housed within the base 20A of housing. Note that the intermediate cover 20B is removed in this figure, so that the line terminal 12, fixed contact 30, moving contact 32 and moving contact arm 34 are visible. The operating mechanism 36 includes an engagement sear 42, shown schematically as a dashed line. The operating mechanism 36 selectively opens and closes the circuit breaker contacts and interacts with the trip unit 50 by engagement of the sear 42 with the pivoting latch 52. As is known to those skilled in the art, latch 52 pivots about a pivoting axis A, sweeping a pivotal motion volume. When the engagement sear 42 and latch 52 are engaged the circuit breaker contacts 30, 32 are maintained in the closed position. Conversely, the contacts are open when the latch 52 and engagement sear 42 are disengaged and the circuit breaker 10 does not enable current flow in the power distribution circuit.
The thermal-magnetic trip unit 50 shown in FIG. 2 includes the latch 52 and latch extension tab 54 that projects laterally from the latch swept volume. As those skilled in the art are aware, the trip unit 50 is of the electromechanical thermal-magnetic type including over current bimetal and an armature assembly that generates a magnetic field attractive to the ferrous metal latch 52. A high current flow through the armature assembly (for example caused by a short circuit in the electrical distribution system) creates a sufficiently dense magnetic flux to pivot the latch 52 in a counterclockwise direction to disengage the operating mechanism sear 42.
FIG. 3A shows the known Siemens circuit breaker 10 second compartment intermediate cover 20B, with the top cover 20C removed to show the fault circuit interrupter unit 27. The intermediate cover 20B defines an aperture 66 for passage of the latch extension tab 54 into the second compartment. The fault circuit interrupter unit 27 includes known fault detection electronics 67 (example: arc fault, ground fault or combination of both) shown schematically and solenoid energizing leads 68. The known Siemens circuit breaker shown in FIGS. 1-3 employs a solenoid 70 (see FIG. 3B) having a magnetically conductive metal solenoid housing 72 about which is wound a coil of conductive wire 74 that is connected to the solenoid energizing leads 68. When the solenoid coil 74 is energized the solenoid 70 generates a torroidal magnetic field that expels metal plunger 76 to the right as shown by the arrow B, where it causes counterclockwise rotation of the latch extension tab 54, thereby disengaging the latch 52 from the engagement sear 42 and causing the operating mechanism 36 to separate the circuit breaker contacts 30, 32. Plunger reset spring 78 resets the plunger to its leftward stable position when the solenoid coil 74 is deenergized.
The known Siemens circuit breaker 10 design provides beneficial separation of the fault circuit interrupter electronics 67 from the compartment containing the moving contacts 30, 32, so that arcs created during contact separation are less likely to contaminate the electronics. Use of the solenoid structure 70 on the left side of the extension tab 54 provides for positive pivoting disengagement of the latch 52 from the operating mechanism sear 42 and leaves open the right side of the extension tab. This is beneficial because trip unit 50 disengagement of latch 52 can be more forceful than that caused by the solenoid, so that the latch is caused to pivot with more counterclockwise rotation. Any components within the circuit breaker housing located to the right of the latch 52 should not impede the latch swept volume space occupied during all operational modes.
Despite the known benefits of the Siemens circuit breaker 10, it is desirable to utilize a latch 52 tripping mechanism in the fault circuit interrupter unit 27 that is simpler and less expensive to manufacture than the prior solenoid 70 designs, yet provides for breaker tripping in a manner harmonious and compatible with the trip unit 50 operational modes.
Other known circuit breakers have utilized electromagnets to trip circuit breakers upon detection of ground and arc fault conditions. As shown in FIG. 4, one other circuit breaker 80 utilizes a pivoting latch 82 that is coupled in series with a second hook 84 that pivots about hoop pivot 85. The hook 84 has a downward projecting tab that abuts against the left side of the latch 82. The hook 84 pivots counterclockwise and in turn pivots latch 82 counterclockwise to disengage the latch and corresponding engagement sear (not shown). Hook 84, constructed of ferrous metal, is urged to pivot in a counterclockwise direction by an electromagnet 86 that attracts the hook upon energization of windings 87 about a bobbin having a ferromagnetic core 88. The serially aligned pivoting latch 82 and hook 84 provide sufficient swept volume space for the latch 82 to be disengaged by the circuit breaker 80 trip unit during overcurrent (bimetal heating) or short circuit trip modes without the electromagnet 86 interfering with latch 82 counterclockwise pivoting motion to the right in the figure. However, utilization of the hook 84 adds an additional component to the circuit breaker design. Also, the need to pivot two serially abutting pivots (latch 82 and hook 84) increases system trip response time or the electromagnet current flux force necessary to move the hook 84 more quickly.
Another known latch mechanism employing an electromagnet is shown in FIGS. 5A and 5B. Circuit breaker 90 has a trip unit 91 that occupies a defined volume within the housing during operational modes. The trip unit includes a known bimetal 92 for overcurrent detection that pivots latch 94 counter clockwise out of engagement with an operating mechanism sear (not shown). An electromagnet comprising a steel core 96 and an annular bobbin/winding 98 capturing the steel core therein provide for combined short circuit and electronic fault detection tripping. During short circuit, the steel core 96 through which the electrical distribution system current passes attracts the latch 94, thereby rotating the latch out of engagement with the operating mechanism. When the electronic fault detection unit sends energizing current into the bobbin/winding 98, the electromagnetic attraction of the armature 94 also causes the breaker to trip. Construction of latch 94 is shown more clearly in the cross sectional view of FIG. 5B. The latch 94 has a generally C-shaped cross section when viewed along the pivot radius, so that it essentially wraps around the bimetal 92. The latch 94 C-shaped cross section must be sufficiently deep left to right, so that the bimetal 92 is afforded its full range of operational deformation and it follows that the range of angular pivot motion of the latch 94 must increase in order to travel additional left-to-right clearance distance. This in turn increases the total occupied volume of the trip unit 91 and impacts the attractive magnetic force strength necessary to pivot the latch during short circuit and electronic fault detection unit trip operational modes. First, there being a limited, finite internal volumetric capacity of any circuit breaker housing, any increase of trip unit volume has adverse impact on other component volume. Second, the C-shaped cross section of the latch 94 increases its mass, thus requiring more current in-rush energy in the coil windings 98 during electronic trip operation or in the steel core 96 during short circuit trip operation to generate a greater magnetic attractive force. Third, the larger pivot angular distance that must be traversed by the latch 94 necessarily increases the distance from the attractive magnetic force of the core 96 and electromagnetic coil 98. The increased distance requires generation of a higher intensity magnetic field in order to generate sufficient attractive force between the latch 94 and the magnetic source.
Thus, a need exists in the art for a trip latch actuator that has simpler construction than known solenoid designs, that does not add additional linkage components to move the trip latch, does not add mass to the trip latch, does not increase the circuit breaker case volume occupied by the trip unit and trip latch, and does not interfere with motion of other parallel-functioning thermal-magnetic trip unit components, such as short circuit armatures or bimetal elements.